This invention relates generally to techniques for processing color television signals and, more particularly, to methods and apparatus for converting color television signals from standard broadcast formats, such as the National Television Systems Committee (NTSC) format, used in the United States, or the Phase Alternation Line (PAL) format, used in much of Europe, to a transfer channel format and, after transmission over a transfer channel, converting the signals to a special output format suitable for reception and decoding by a conventional PAL receiver.
In video tape recording, television signals in the standard broadcast formats such as NTSC or PAL are unsuitable for recording directly on magnetic tape, because time base variations, which inevitably occur in the signal recovered from the tape, create unacceptable crosstalk between the various signal components. Hence, video tape recorders usually include apparatus for converting the broadcast format to a suitable transfer channel format, and for reconverting back to a broadcast format again, in order that the recorded information can be reproduced in a conventional television receiver. The present invention is principally concerned with such apparatus for use in situations in which the originally received signal (to be recorded) is in either NTSC or PAL format, and the conventional receiver used in the reproduction of the recorded signals is a PAL-type receiver.
First, by way of background, both NTSC and PAL format color television signals include a baseband luminance signal, an audio subcarrier and a quadrature modulated chrominance subcarrier. For NTSC, the chrominance subcarrier comprises one component at a phase angle of +33.degree., modulated by a first chrominance signal Q, and a quadrature component at a phase angle of +123.degree., modulated by a second chrominance signal I. The signals Q and I are derived from color difference signals B-Y and R-Y, where B is a signal representative of the color blue, R is a signal representative of the color red, and Y is a signal representative of luminance, in accordance with the following formulas: EQU Q=0.41(B-Y)+0.48(R-Y) EQU I=-0.27(B-Y)+0.74(R-Y)
The PAL-format chrominance subcarrier, on the other hand, comprises one component at a phase angle of 0.degree., modulated by a color difference signal U, which is defined to be 0.493(B-Y), and a quadrature component at a phase angle that alternates on successive horizontal scan lines between +90.degree. and -90.degree., modulated by a color difference signal V, which is defined to be 0.877(R-Y).
Chrominance bursts, which serve as a phase and frequency reference for the quadrature chrominance components, occur during the horizontal blanking intervals. NTSC bursts all occur at a phase angle of 180.degree., and PAL bursts alternate on successive horizontal lines between phase angles of +135.degree. and -135.degree..
In the aforementioned co-pending application, there are disclosed a number of color television signal encoding devices employing techniques of the general type described above, for processing NTSC or PAL format signals. Each device disclosed in the prior application includes a recording subsystem to convert a television signal input to a special transfer channel format for insertion on a transfer channel such as magnetic recording tape, and a player subsystem to convert the signal recovered from the transfer channel substantially back to its original format. This special transfer channel format of the television signal placed on the transfer channel is the same for all the disclosed devices and includes frequency modulated luminance and audio subcarriers and a bi-sequential chrominance subcarrier, which is amplitude modulated with suppressed subcarrier on alternate horizontal lines, first by the color difference signal U (or 0.493(B-Y)) and then by the other color difference signal V (or 0.877(R-Y)).
In the recording subsystems of the disclosed devices, including both those for processing NTSC-format signals and those for processing PAL-format signals, the special transfer channel chrominance subcarrier is produced by mixing the broadcast format quadrature subcarrier with a locally generated reference signal that is derived from a reference having the same frequency as the quadrature subcarrier and having a phase angle switched on alternate horizontal scan lines between 0.degree. and +90.degree.. When the phase angle of the local reference is 0.degree., a chrominance subcarrier modulated by U is produced, and when the phase angle of the local reference is +90.degree., a subcarrier modulated by V is produced. For the disclosed PAL devices, the alternate lines in which the local reference has a phase angle of +90.degree. must be synchronized with the alternate broadcast lines that includes a component modulated by +V, as contrasted with -V.
It will be noted that in all of the devices disclosed, the transfer channel chrominance subcarrier includes only one of the color difference signals, i.e., U or V, for each horizontal scan line, whereas the quadrature chrominance subcarrier to be re-created (PAL or NTSC) includes separate U and V components that are present simultaneously in each line. As a result, the player subsystems of each device must combine successive lines of chrominance information in the signal recovered from the transfer channel, to re-create the proper quadrature format.
In the player subsystems of the devices for processing NTSC-format signals, the quadrature chrominance subcarrier is re-created by first converting the line-sequential transfer channel chrominance subcarrier recovered from the transfer channel to a corresponding line-sequential subcarrier at the original quadrature subcarrier frequency. The phase angle of this latter subcarrier is switched between 0.degree. for horizontal scan lines modulated by U and +90.degree. for lines modulated by V. This switched phase subcarrier is then passed through a delay line to delay it by one horizontal line period, and, in turn, combined in a summing amplifier with the non-delayed switched phase subcarrier, to produce the NTSC-format quadrature chrominance subcarrier. Proper NTSC-format chrominance bursts are produced by summing in bursts having a phase angle of 180.degree. during the horizontal blanking intervals.
Similarly, in the player subsystems of the devices for processing PAL-format signals, the quadrature chrominance subcarrier is re-created by converting the transfer channel chrominance subcarrier to a corresponding line-sequential subcarrier having the same frequency as the original quadrature subcarrier and having a phase angle switched between 0.degree. for lines modulated by U and +90.degree. for lines modulated by V. The devices also include a delay line for delaying the switched phase subcarrier by one horizontal line period, an inverter and switch combination for inverting the phase of the delayed signal on alternate horizontal lines, and a summing amplifier for combining the delayed subcarrier from the inverter and switch combination with the non-delayed subcarrier, to produce the quadrature chrominance subcarrier.
The inverter and switch combination is required in the PAL devices described above because the phase angle of the V component of the PAL-format quadrature chrominance subcarrier to be re-created must be switched on alternate lines between +90.degree. and -90.degree.. For those lines in which the line-sequential chrominance subcarrier being presently received is modulated by V, the switch automatically selects the non-inverted delayed signal (i.e. +U) to be supplied to the summing amplifier, whereby a quadrature subcarrier modulated by +U and +V is produced. On the other hand, for the alternate lines, in which the line-sequential chrominance subcarrier being presently received is modulated by U, the switch automatically selects the inverted delayed signal (i.e. -V) to be supplied to the summing amplifier, whereby a quadrature subcarrier modulated by +U and -V is produced.
Proper PAL-format chrominance bursts are produced in the PAL processing devices by summing the line-sequential switched phase chrominance subcarrier with bursts having a 180.degree. phase angle during the horizontal blanking intervals of the lines modulated by +U and bursts having a +90.degree. phase angle during the blanking intervals of the lines modulated by +V. By this technique, the summing amplifier will produce bursts having phase angles of +135.degree. for those lines inwhich the inverter and switch combination produces a non-inverted delayed signal (and the quadrature subcarrier thereby produced is modulated by +U and +V), and -135.degree. for the alternate lines in which the inverter and switch combination produces an inverted delayed signal (and the quadrature subcarrier thereby produced is modulated by +U and -V).
A major drawback to the aforedescribed devices for processing PAL-format color television signals arises because of their inclusion of a delay line in re-creating the quadrature modulated chrominance subcarrier. The inclusion of such delay lines necessarily increases both the complexity and the cost of the devices. Additionally, inclusion of a delay line creates a mis-registration of luminance and chrominance information in the video picture reproduced by a conventional PAL-type color television receiver. This results because half of the chrominance information is always delayed by one horizontal line period, while the luminance information is not delayed at all. Although such devices have provided satisfactory performance in some instances, there is still a need for a less complex and less expensive alternative that will provide improved performance.
Another drawback to the aforedescribed devices arises from inherent characteristics of the PAL-format and NTSC format signals, themselves. The maximum amplitude for the transfer channel chrominance subcarrier when modulated by the color difference signal V is about 3 db greater than its maximum amplitude when modulated by the color difference signal U. This results both in wasted dynamic range of the chrominance circuitry and a substantially lower signal-to-noise ratio for those alternate horizontal lines in which the lower level color difference signal is being produced.
An additional drawback affecting the devices that process PAL-format signals, arises because proper chrominance bursts are produced in the quadrature modulated subcarrier output only by summing together separate bursts occurring in successive horizontal blanking intervals of the bi-sequentially modulated subcarrier. As a result, it is not until the second horizontal line of chrominance information in each field that a burst with the proper phase will be produced. Accordingly, the quality of the color in a video picture reproduced from the color reproduction achieved by a conventional PAL-type television receiver to which such a prior device is coupled, may be adversely affected, at least to a limited degree.
It will be appreciated from the foregoing that there is still a need for improved apparatus for converting a standard broadcast format television signal to a transfer channel format and, in turn, to a format suitable for reception and decoding by a conventional PAL-type receiver, wherein such apparatus will provide proper chrominance bursts for all the horizontal lines in each frame, and potentially will utilize substantially the entire dynamic range of the chrominance circuitry on every horizontal line. Additionally, there is still a need for apparatus of this type that can be constructed without undue complexity or cost and, in particular, without the need for a delay line for delaying a signal by one horizontal line period. The present invention fulfills these needs.